ScheduledExecutorService with variable delay

Question

Suppose I have a task that is pulling elements from a java.util.concurrent.BlockingQueue and processing them.

public void scheduleTask(int delay, TimeUnit timeUnit)
{
    scheduledExecutorService.scheduleWithFixedDelay(new Task(queue), 0, delay, timeUnit);
}

How can I schedule / reschedule the task if the frequency can be changed dynamically?

• The idea is to take a stream of data updates and propagate them in batch to a GUI
• The user should be able to vary the frequency of updates

Answer 1

Shouldn't you be using scheduleAtFixedRate if you are trying to process several queue tasks with a specific interval? scheduleWithFixedDelay will only wait for the specified delay and then execute one task from the queue.

In either case, the schedule* methods in a ScheduledExecutorService will return a ScheduledFuture reference. If you want to change the rate, you can cancel the ScheduledFuture and reschedule the task with a different rate.

Answer 2

I don't think you can change a fixed rate delay. I think you need to use schedule() to perform a one-shot, and schedule again once that has completed (with a modified time out if required).

Answer 3

scheduleWithFixedDelay(...) returns a RunnableScheduledFuture. In order to reschedule it, you might just cancel and reschedule it. To reschedule it, you may just wrap the RunnableScheduledFuture wit a new Runnable:

new Runnable() {
    public void run() {
        ((RunnableScheduledFuture)future).run();
    }
};

Answer 4

Use schedule(Callable<V>, long, TimeUnit) rather than scheduleAtFixedRate or scheduleWithFixedDelay. Then ensure that your Callable reschedules itself or a new Callable instance at some point in the future. For example:

// Create Callable instance to schedule.
Callable<Void> c = new Callable<Void>() {
  public Void call() {
   try { 
     // Do work.
   } finally {
     // Reschedule in new Callable, typically with a delay based on the result
     // of this Callable.  In this example the Callable is stateless so we
     // simply reschedule passing a reference to this.
     service.schedule(this, 5000L, TimeUnit.MILLISECONDS);
   }  
  }
}

service.schedule(c);

This approach avoids the need to shut down and recreate the ScheduledExecutorService.